The present invention relates to an improved method for patterning magnetic data/information recording, storage, and retrieval media as well as improved magnetic recording media obtained thereby. More specifically, the present invention relates to an improved method for patterning magnetic media in hard disk form such as are utilized in computer and computer-related applications.
Magnetic media are widely utilized in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are typically formed as xe2x80x9cperpendicularxe2x80x9d or xe2x80x9clongitudinalxe2x80x9d media depending upon the direction of magnetization of the grains. In this regard, xe2x80x9cperpendicularxe2x80x9d recording media have been found superior to the more common xe2x80x9clongitudinalxe2x80x9d media in achieving very high bit densities. However, as grain sizes decrease in order to achieve increased recording bit densities, e.g., to about 20 Gb/in2, effects arising from thermal instability, such as xe2x80x9csuperparamagnetismxe2x80x9d are encountered. One proposed solution to the problem of thermal instability, including, inter alia, the so-called xe2x80x9csuperparamagnetic limitxe2x80x9d encountered with ultra-high recording density magnetic recording media, is to increase the crystalline anisotropy, and thus the squareness of the bits, in order to compensate for the smaller grain sizes.
An alternative approach, however, to the formation of very high bit density magnetic recording media, is the formation of xe2x80x9cpatternedxe2x80x9d media. Analogous to the situation with conventional polycrystalline thin-film magnetic media, both xe2x80x9clongitudinalxe2x80x9d and xe2x80x9cperpendicularxe2x80x9d types of patterned magnetic media have been developed, depending upon whether the magnetization direction is parallel or perpendicular to the media surface. When fabricated in disk form, such xe2x80x9cpatternedxe2x80x9d media are readily adapted for use in conventional hard drives, with most of the drive design features remaining the same. Thus, hard drive-based xe2x80x9cpatternedxe2x80x9d media technology would, in practice, comprise a spinning disk with a slider head flying above it in closely-spaced relation thereto, with read sensors or a read/write head that magnetizes and/or detects the magnetic fields emanating from the magnetic media.
According to a first approach, as exemplified by the Atomic Force Microscopy (xe2x80x9cAFMxe2x80x9d) approach of IBM (B. Terris et al., Data Storage, August 1998, pp. 21-26), a sharp tip is utilized for scanning extremely close to the surface of a data/information storage medium. The tip is located at the end of a flexible cantilever, which deflects in response to changes in the force imposed on the tip during scanning. The force may arise from a variety of effects, including, inter alia, magnetic force. To date, only two types of AFM drives have been demonstrated, i.e., write-once/read-only and read-only. The former type of AFM drive, which provides write-once/read-only capability, utilizes a heated AFM tip for writing once by forming small indentations or pits in the surface of a substrate, e.g., of polycarbonate. Data is read by using the AFM tip to scan the thus-indented surface and sensing the changes in the force imposed on the AFM tip due to the presence of the indentations.
The latter type of AFM drive functions in a read-only mode, and data is initially written in the form of indentations (pits) which are created in the surface of a SiO2 master by means of an electron beam. The data, in the form of the indentations, is then transferred, by replication, to a photopolymer-coated glass substrate, which photopolymer is cured by exposure to ultra-violet (UV) radiation to thereby form a surface topography representing the data. The data is then read from the cured photopolymer surface by scanning with the AFM tip to sense the changes in force thereat due to the indentations.
According to the second, lithographic approach, thin film processes such as are utilized in the fabrication of semiconductor integrated circuits including micron-sized features are adapted for making high aspect ratio, single column/bit, perpendicularly patterned media. According to one particular approach (M. Todorovic et al., Data Storage, May 1999, pp. 17-20) designed to increase coercivity, hence stability of the individual columns, electroplated nickel (Ni) is utilized for forming the columns, and gallium arsenide (GaAs) and alumina (Al2O3) are employed as embedding media for the columns. The fabrication process starts with an electrically conductive GaAs substrate, on which thin layers of aluminum arsenide (AlAs) and GaAs are successively deposited, as by molecular beam epitaxy (xe2x80x9cMBExe2x80x9d). Scanning electron beam lithography is then utilized to define the magnet patterns on the resin-coated sample. The patterns in the e-beam-exposed resin are developed utilizing an appropriate solvent system and then transferred, as by chemically-assisted ion beam etching (xe2x80x9cCAIBExe2x80x9d), into the AlAs/GaAs layers. After pattern definition, the AlAs layer is converted into Al2O3 by wet thermal oxidation. The thus-produced patterned layer acts as a mask for additional etching for extending the pattern of depressions perpendicularly into the GaAs substrate. The etched depressions in the Al2O3 substrate are then filled with electroplated Ni. Overplated Ni xe2x80x9cmushroomsxe2x80x9d are then removed, as by polishing, to create a smooth surface for accommodating slider contact therewith.
Thus, the overall process sequence for forming such media requires successive, diverse technology steps for (1) MBE growth and mask deposition; (2) electron beam lithography; (3) chemically-assisted ion beam etching; (4) wet thermal oxidation; and (5) electroplating and polishing, e.g., chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d). The result is a complex and time-consuming fabrication process. Moreover, each of the above-described approaches for patterned media manufacture typically involves substantial capital investment for the process equipment, which together with the inherent process complexity, render them too costly for use in high product throughput magnetic disk media manufacture.
Yet another process for forming patterned magnetic media, which process is also useful in forming servo patterns on a magnetic media surface, is disclosed by D. S. Kuo in commonly assigned, co-pending U.S. patent application Ser. No. 09/130,657, filed Aug. 7, 1998, and is based upon the well-known property or phenomenon of magnetic films of exhibiting a decrease in coercivity (Hc) with increase in temperature. Such decrease in Hc, with increase in temperature is currently utilized to produce magnetic transitions in thermomagnetic materials, e.g., rare earth-transition metal (xe2x80x9cRE-TMxe2x80x9d) materials, such as terbium-iron (TbFe) films utilized in magneto-optical (xe2x80x9cMOxe2x80x9d) recording devices. Such devices typically employ a focussed laser beam for creating a xe2x80x9chot spotxe2x80x9d on the RE-TM-based media surface, while simultaneous application of an external magnetic field is applied to the media to reverse the direction of local magnetization within the locally heated area.
Based upon this effect or phenomenon, Kuo has proposed, in the above-mentioned U.S. patent application, a method for forming patterned magnetic media, e.g., servo patterns in the surface of a magnetic recording layer. According to the process disclosed therein, instead of heating the magnetic media with a spot of focussed laser radiation, a focussed pattern (i.e., an image) of radiative energy (e.g., from a laser) is projected onto the surface of a magnetic recording film or layer, which film or layer has been subjected to a pre-alignment treatment by application of a strong magnetic field of a first polarity, to selectively heat and thus lower the coercivity Hc of the magnetically pre-aligned film at the exposed areas. In order to generate a magnetic pattern in the magnetically pre-aligned film or layer, a reverse polarity, weaker magnetic field is applied (from an external source) to the film surface during the exposure/heating process, the reverse polarity, weaker magnetic field having a strength between the coercivity of the magnetic film when at ambient room temperature (i.e., when cold) and when under selective radiative energy exposure (i.e., when hot). The direction or orientation of magnetization of the magnetic film or layer at the selectively heated areas corresponding to the exposure pattern is reversed due to the application of the reverse polarity magnetic field during the pattern exposure/selective heating stage. As a consequence, a magnetic pattern is formed in the magnetic film or layer, which pattern is retained upon subsequent cooling of the selectively exposed portions of the film or layer.
While the feasibility of the above concept or process has been demonstrated in laboratory studies, obtainment of patterns exhibiting high quality magnetic transitions has been problematic, for several reasons, including:
(1) Transition sharpness and signal strength-adaptation of the essential data recording concept utilized with RE-TM thermomagnetic materials and media, such as of TbFe, to magnetic recording media utilizing thin films or layers of conventional magnetic alloys comprised of iron (Fe), cobalt (Co), nickel (Ni), chromium (Cr), platinum (Pt), etc., incurs a complication in that although the coercivity Hc of the latter-mentioned magnetic materials decreases with increase in temperature, the anisotropy constant Ku also decreases with increase in temperature. As a consequence, the combined effect of a decrease in Hc and Ku with increase in temperature is disadvantageous in several respects, including formation of wide transitions between the patterned and non-patterned areas; a wide pulse width at 50% of peak signal amplitude (xe2x80x9cPW 50xe2x80x9d); and weak signal strength.
(2) Magnetic pattern uniformityxe2x80x94the shape of the transition patterns between areas of different magnetic orientation is determined by the temperature distribution of the selectively heated areas of the magnetic film or layer resulting from the heating by radiative (e.g., laser) energy. The temperature vs. distance along the film surface profile can vary from area-to-area due to several factors. For example, the radiative energy supplied to the various selected areas of the pattern can vary due to pulse-to-pulse energy variation of the laser, such that even if the image geometry is quite uniform over the film surface, the resulting magnetic transition patterns can exhibit substantial variation is both size and shape. In addition, since the coercivity Hc, of as-deposited magnetic alloy films or layers typically varies by about +/xe2x88x9210% over a disk surface, pattern uniformity and signal strength will necessarily vary over the magnetic layer area.
(3) Dynamic coercivityxe2x80x94the magnetic switching or transition time is determined primarily by the length of the cooling interval of the thermal cycle (i.e., the combination of heating and cooling times) experienced by the magnetic film or layer. In order to form a magnetic pattern with sharp magnetic transitions, it is necessary to generate a heating pattern of the selected areas which has a large temperature gradient near the edges thereof. To achieve this, a short thermal cycle is required in order to minimize expansion (e.g., widening) of the temperature profile resulting from thermal conduction in the magnetic film or layer. However, as the duration of the thermal cycle decreases, the coercivity of the magnetic film or layer corresponding to the cycle (i.e., switching) time, termed the xe2x80x9cdynamic coercivityxe2x80x9d, increases. Consequently, an upper limit is imposed on how rapidly the magnetic film or layer can be heated without incurring significant dynamic coercivity effects. The same upper limit determines the minimum amount of expansion (i.e., widening) of the temperature vs. distance along the film surface profile for a finite heating interval.
Accordingly, there exists a need for an improved method for forming magnetic patterns in magnetic data/information storage and retrieval media, such as hard disks, which is free of the disadvantages and drawbacks associated with the above-described process, and which can be implemented at a manufacturing cost which is lower than, or at least compatible with, that of conventional manufacturing methodologies and technologies for forming patterned magnetic media. There also exists a need for improved patterned magnetic media, e.g., in disk form, which exhibit uniformly shaped and very sharply defined magnetic patterns, such as are employed for servo patterns of thin film magnetic media.
The present invention, therefore, addresses and solves problems attendant upon patterned magnetic media manufacture according to the above-described process, and affords rapid, cost-effective fabrication of high bit density, patterned magnetic media, e.g., in the form of hard disks, while providing substantially full compatibility with all mechanical and electrical aspects of conventional hard disk technology. Moreover, the patterned magnetic media of the present invention can be simply and reliably manufactured by suitable adaptation and/or modification of conventional manufacturing techniques and apparatus.
An advantage of the present invention is an improved method of forming a magnetic pattern in a magnetic data/information storage and retrieval medium.
Another advantage of the present invention is an improved magnetically patterned magnetic data/information storage and retrieval medium.
Yet another advantage of the present invention is an improved apparatus for forming a magnetic pattern in a magnetic data/information storage and retrieval medium.
Additional advantages, aspects, and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in part by a method of forming a magnetic pattern in a magnetic data/information storage and retrieval medium, which method comprises the steps of:
(a) providing a magnetic medium including a magnetic recording layer having a surface, the magnetic recording layer comprising a magnetic material having a Curie temperature (Tc) substantially above room temperature;
(b) applying to the magnetic layer a first uniform magnetic field of a first direction and strength sufficient to substantially uniformize the magnetic state thereof;
(c) applying to the magnetic layer a second uniform magnetic field of a second direction opposite to and of lesser strength than that of the first magnetic field, the strength of the second magnetic field being sufficient to alter the magnetic state of the magnetic layer when the latter is at a first, elevated temperature equal to or greater than Tc but insufficient to alter the magnetic state of the magnetic layer when the latter is at a second, lower temperature below Tc;
(d) selectively increasing the temperature of at least one selected portion of the surface area of the magnetic layer to at least Tc for a desired interval, while applying the second magnetic field to the magnetic layer, thereby selectively altering the magnetic state of the at least one selected portion of the magnetic layer;
(e) terminating application of the second magnetic field to the magnetic layer after completion of step (d); and
(f) cooling the at least one selected portion of the magnetic layer to a temperature below Tc.
According to embodiments of the present invention, step (a) comprises providing a longitudinal or perpendicular magnetic medium, e.g., a disk-shaped medium including a disk-shaped substrate supporting the magnetic layer, wherein the magnetic layer is comprised of a magnetic alloy including metals selected from the group consisting of Fe, Co, Ni, Cr, and Pt, and Tc of the magnetic alloy is on the order of about 300xc2x0 C.; step (b) comprises substantially uniformizing the magnetic state of the magnetic layer by pre-aligning the magnetic regions thereof, e.g., by performing a DC erase by applying the first uniform magnetic field as a DC magnetic field directed along the easy axis of magnetization of the magnetic layer, the magnetic field strength of the first magnetic layer being substantially greater than the Dc coercivity of the magnetic layer at room temperature and the first magnetic field being applied to the entire surface of the magnetic layer for a desired interval; step (c) comprises applying the second uniform magnetic field of direction opposite to the first magnetic field at a strength which is lower than but close to that where lowering of the magnetization of the magnetic layer occurs as the temperature of the magnetic layer approaches Tc and may, for example, be determined by use of the following equation for magnetic materials with constant squareness (S) values:                               H          c                ⁡                  (                                    ⅆ              M                        /                          ⅆ              H                                )                    Hc        -          M      r                          (                              ⅆ            M                    /                      ⅆ            H                          )            Hc        -                  (                              ⅆ            M                    /          dH                )            0      
where Hc=coercivity (Oe); M=magnetization (emu/mm3); Mr=remanent magnetization; and subscript 0=zero applied magnetic field; step (d) comprises raising the temperature of a patterned plurality of selected portions of the surface of the magnetic layer to at least Tc, e.g., by selectively exposing the patterned plurality of portions of the magnetic layer to radiative energy, by generating a pattern of radiative energy for selectively exposing the patterned plurality of portions by projecting a focussed image on the selected portions of the surface of the magnetic layer via photolithographic techniques, by contact/proximity imaging through a patterned aperture mask, or by near field imaging, wherein step (d) comprises utilizing a source of radiative energy selected from among optical (e.g., continuous or pulsed lasers), electron beam, and ion beam sources and performing raster scanning or area imaging of the magnetic layer surface; and step (e) comprises rapidly cooling the at least one selected portion by at least one process selected from convection, conduction, and heat transfer.
According to another aspect of the present invention, a patterned magnetic medium comprises:
(a) a substrate; and
(b) a patterned magnetic recording layer on the substrate, the patterned magnetic recording layer having a Curie temperature (Tc) substantially above room temperature and a surface comprising a magnetic pattern formed therein by a process comprising the steps of:
i. applying to the easy axis of the magnetic layer a first uniform magnetic field of a first direction and having sufficient strength to substantially uniformize the magnetic state of the magnetic layer by pre-alignment of the magnetic regions thereof;
ii. applying to the magnetic layer a second uniform magnetic field of a second direction opposite to and having a strength less than that of said first magnetic field, the strength of the second magnetic field being sufficient to alter the alignment of the magnetic regions of the magnetic layer when the latter is at a first, elevated temperature equal to or greater than Tc but insufficient to alter the alignment of the magnetic regions of the magnetic layer when the latter is at a second, lower temperature below Tc;
iii. selectively increasing the temperature of a patterned plurality of selected portions of the magnetic layer to at least Tc for a desired interval while applying the second magnetic field to the magnetic layer, thereby selectively altering the magnetic alignment of the magnetic regions of the patterned plurality of selected portions of the magnetic layer;
iv. terminating application of the second magnetic field to the magnetic layer after completion of step iii.; and
v. cooling the patterned plurality of selected portions of the magnetic layer to a temperature below Tc.
According to embodiments of the present invention, the patterned magnetic medium comprises a disk-shaped substrate and a longitudinal or perpendicular magnetic recording layer comprised of an alloy of elements selected from Fe, Co, Ni, Cr, and Pt and having a Tc on the order of about 300xc2x0C.
According to yet another aspect of the present invention, a system for forming a magnetic data/information storage and retrieval medium having a patterned magnetic recording layer comprises:
means for applying a magnetic field to a magnetic recording layer having a Curie temperature (Tc) which is substantially above room temperature; and
means for forming a magnetic pattern in said magnetic layer by selectively increasing the temperature of a selected plurality of portions of the magnetic layer to at least Tc for a desired interval.
According to embodiments of the present invention, the means for applying a magnetic field to the magnetic recording layer comprises magnet means for applying a uniform magnetic field of a desired direction and having a strength sufficient to alter the magnetic state of the magnetic layer when the latter is at a first, elevated temperature at or above Tc but insufficient to alter the magnetic state of the magnetic layer when the latter is at a second, lower temperature below Tc; and the means for forming the magnetic pattern includes a radiative energy source for selectively increasing the selected plurality of portions of the magnetic layer to a temperature of Tc or above.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as limitative.